Methacrylic resins have mechanical properties, molding-processability, weather-resistance and the like in good balance, and are used in a variety of fields as sheet materials or molding materials. Further, methacrylic resins also have excellent optical properties such as transparency, low dispersion, low birefringence and the like. Recently, methacrylic resins have, utilizing such properties, widespread uses such as disk materials of video disks, audio disks, unrewritable disks used in computers, lens materials of video cameras, projection type televisions, light pickup lenses and the like, and various light transmission materials such as optical fibers, optical connectors and the like.
However, methacrylic resins have problems that hygroscopic property is high and heat-resistance is low. Namely, a molded article made of methacrylic resin exhibits size variation and warping by moisture absorption and manifests cracking by repeated cycles of moisture absorption and drying for a long period, and use of methacrylic resins is restricted for some specific articles. Such problems are said to be significant particularly for disk materials, and light pickup lenses and connectors and the like used in such optical systems. Further, use of methacrylic resins in articles used in automobiles is sometimes restricted due to low heat-resistance. Moreover, the same problems are found in acrylic sheets. Recently, lower birefringence is required in optical resin materials such as disk materials, lens materials and the like due to higher density of recording media, and the birefringence of polymethyl methacrylate (hereinafter, abbreviated as PMMA) may be insufficient in some specific fields.
Accordingly, there have been many suggestions recently for improvement of the hygroscopic property, enhancement of heat-resistance, lower birefringence and the like while retaining the optical properties of methacrylic resins. For example, for methacrylic resin less hydrophic, there have been suggested a copolymer composed of methyl methacrylate and cyclohexyl methacrylate (Japanese Patent Application Laid-Open (JP-A) No. 58-5318), and a copolymer composed of methyl methacrylate, cyclohexyl methacrylate and benzyl methacrylate (JP-A No. 58-13652). However, they have a demerit of lowered heat-resistance even though the low hygroscopic property is improved. Further, for imparting heat-resistance and low birefringence, there have been suggested a copolymer composed of methyl methacrylate and o-methylphenylmaleimide (JP-A No. 60-217216) and a copolymer composed of methyl methacrylate and a maleimide compound (JP-A No. 61-95011). However, also in these cases, the copolymers have such demerits that coloration is generally significant, due to introduced maleimide-based monomers.
On the other hand, there is a method in which heat-resistance is improved by suppressing molecular motion via use of a hard polymer chain. A polymer having a backbone pyran ring has been suggested as such polymer (U.S. Pat. No. 4,899,948 and Lon J. Mathias. Polymer. 35(15). 3317. 1994). However, this polymer is made of a methyl ester, isobonyl ester, trimethylcyclohexyl ester and the like, and has fairly high birefringence and can not be subjected to usual injection molding due to low mechanical strength. Further, the copolymer disclosed in this literature is a cross-linked polymer and a molded article thereof is not thermally melted and consequently melt molding is impossible. Also, a copolymer has been suggested which can be melt-molded owing to increased monomer purity (U.S. Pat. No. 5,247,035).
Further, a transparent thermoplastic resin having high ability is required in various fields including optical used and automobile use. Among others, materials of lenses for laser beam printers are required to have various properties further increased in order to obtain clear images. Particularly, low hygroscopic property, low birefringence and high heat-resistance are most important properties.
At present, glass and plastic materials are mainly used as lens materials. However, glass materials can not be recognized to have excellent mechanical strength, mass-productivity and cost performance.
Under such circumstance, a transparent thermoplastic resin, PMMA is frequently used as a lens material now in view of its strength and productivity. Though this PMMA has low birefringence, it has problems of high hygroscopic property and low heat-resistance. That is, because of size variation caused by water absorption, strain is formed in projecting a laser beam to a drum and quality of images tends to lower. Size variation is also caused by heat.
Since the beginning of the market for compact disks (CD) and laser disks (LD) at the beginning of 1980s, optical disks have been in steep increase. Recently, it has become desirable to record in digital mode dynamic images having a volume corresponding to LD in an optical disk in CD size, and a variety of thin disks having raised density are in development (lectures 17p-T-11 and 17p-T-13 in The Society of Applied Physics (Oyo butsuri Gakkai), lectures 29-a-B-8 and 29a-B-5 in The Society of Applied Physics, and the like).
Substrates of such optical disks are molded by an injection molding method which is inexpensive and capable of mass production, and PMMA and polycarbonate have been suggested as resin materials for substrates.
Regarding the plastic optical fiber (hereinafter, referred to as "POF"), PMMA is mainly used as a core material thereof since it has low transmission loss and mechanical properties and weather-resistance thereof present no problem. Upper limit temperature in use of an optical fiber containing PMMA as a core material is at most about 105.degree. C. even when this fiber is endowed with coating excellent in weather-resistance, and heat-resistance thereof is insufficient for communication in movable bodies such as automobiles, electric cars, airplanes and the like and for outdoor use.
For improving heat-resistance of POF, there is a method in which a core material having high Tg is used. As the core material having high Tg, there are known 1) a material comprising a polycarbonate which itself exhibits high glass transition temperature (JP-A No. 61-262706), 2) a material comprising an olefin-based copolymer containing polycyclic olefin-based monomer (JP-A No. 61-211315), 3) a material comprising a methyl methacrylate/aromatic maleimide copolymer (Japanese Patent Application Publication (JP-B) Nos. 5-82405 and 5-82406), 4) a material comprising a methyl methacrylate/aliphatic maleimide copolymer (JP-A No. 63-80205), and 5) a material comprising a methyl methacrylate/alicyclic methacrylate copolymer (JP-A No. 61-260205).
For illuminating a meter panel mounted on a vehicle, airplane and the like, a light-inductive material made of an inorganic glass or transparent resin material is used. This light-inductive material carries a side face as a receptive surface, and a light emitted from a light source placed near the reception surface is introduced as an incident light into the light-inductive material and reflected on a reflection layer such as an aluminum deposited layer and the like provided on the rear surface of the light-inductive material, to illuminate indications situated on an emission surface on the light-inductive material.
It is usual for a light-inductive material for the light source to be placed directly near a reception surface so that a light having a quantity sufficient for illumination of an emission surface on the front side is allowed to pass through the reception surface having a small area, and it is necessary that the reception surface is not deformed or degraded by heat generated from the light source.
A light-inductive material made of an inorganic glass has a demerit of poor processability though it has excellent heat-resistance and transparency, therefore, recently light-inductive materials made of thermoplastic polymers are often used.
There are acrylic polymers, typified by PMMA, which are thermoplastic polymers having excellent transparency and weather-resistance. However, they have a demerit that light-inductive materials molded from these polymers tend to be deformed by heat generated in lighting of a lamp.
Polycarbonate resins generally have high impact-resistance and transparency, and sheets extrusion-molded are widely used in industrial fields such as a sound insulation walls, signboards and the like because of these physical properties. When a polycarbonate sheet is singly used, weather-resistance is poor. For improving this poor weather-resistance, there are known a method in which a film made of a methacrylic resin excellent in weather-resistance containing an ultraviolet absorber is laminated on the surface of a polycarbonate sheet and a method in which the surface of a polycarbonate sheet is treated by clear coating. In these methods, lamination methods are truly industrialized, but are difficult in cost reduction and improvement of productivity.
For realizing the reduction in cost and improvement of productivity, a co-extrusion method is known. By this method, a methacrylic resin can be directly laminated on a polycarbonate resin in a die of an extruder.
Methacrylic resins have demerits of low impact-resistance and heat-resistance. Therefore, many attempts have been made regarding improvement of impact-resistance, enhancement of heat-resistance and the like while retaining weather-resistance and optical properties of methacrylic resins.
Impact-resistance of a methacrylic resin can be improved by adding a reinforcing additive based on an elastomer material. In general, this additive is a polymer having multilayered particle structure, and at least one layer thereof is composed of an elastomer phase and refractive index thereof is controlled so that a transparent material may be obtained. However, when impact-resistance is increased, optical properties, flexural strength and heat-resistance are lowered.
As polymers having improved heat-resistance, random polymers are public-known such as a methyl methacrylate polymer chemically modified via imidation, a methyl methacrylate/.alpha.-methylstyrene/N-cyclohexylmaleimide copolymer, and the like. However, these polymers have low impact-resistance. Further, these polymers have high refractive index, therefore, for obtaining a transparent material having enhanced impact-resistance, only an additive having an elastomer phase based on a polybutadiene poor in weather-resistance can be used. Furthermore, in these cases, it is known that a yellowish color is exhibited when heat-melt-molding is effected due to introduction of an imide-based component. For solving this problem, an attempt has been made to suppress oxidative degradation by addition of an antioxidant, but the resulting effects were insufficient.
Further, as a thermoplastic polymer having relatively excellent transparency and high heat-resistance temperature, polycarbonates are envisaged. Since polycarbonates have a problem of poor weather resistance, there are known, for outdoor use thereof, a method in which an ultraviolet ray absorber is added to a polycarbonate and a method in which a film made of a methacrylic resin containing an ultraviolet ray absorber is laminated on the surface of a polycarbonate. However, these methods can not provide weather-resistance corresponding to that of PMMA. Namely, a heat-resistant resin having oxidation-degradation-resistance and weather-resistance corresponding to those of PMMA can not be obtained in current conditions.
Also, there has been an attempt for improving impact-resistance and heat-resistance simultaneously. This is related to a composition comprising a syndiotactic methyl methacrylate polymer and a reinforcing additive (JP-A No. 6-287398), however, a method for producing this polymer can not be easily industrialized and impact-resistance and heat-resistance of the composition are not sufficient.
Front surface of a head lamp and fog lamp of an automobile and a signal lamp is equipped with a lamp lens usually made of an inorganic glass for retaining light permeability. However, inorganic glasses have demerits of cracking tendency, heavy weight and the like though they are excellent in heat-resistance, transparency and weather-resistance. Accordingly, in some fields, a lamp lens made of a plastic has been developed and actually used.
Thermoplastic polymers excellent in transparency and weather-resistance include acrylic polymers represented by PMMA. However, lamp lenses molded from these polymers have a demerit of deformation tendency due to heat generated in lighting of a lamp. Further, as a thermoplastic polymer having relatively excellent transparency and weather-resistance and high heat-resistant temperature, polycarbonates are envisaged.
Light scattering sheet materials obtained by molding acrylic resins by adding a light scattering agent are used in illumination cover, signboard, display, glazing uses as well as other uses as materials for various molded articles having soft feeling unique to the materials. Recently, development of a high performance heat-sensitive material having both high light permeability and high light scattering property is desirable particularly in the illumination field.
In the past, various means have been used for imparting light scattering property, and among others, compositions into which organic fillers such as barium sulfate, calcium carbonate, silicon dioxide, talc, titanium dioxide, aluminum hydroxide and the like or power of resins having different refractive indices are compounded are exemplified.
However, a composition obtained by compounding an inorganic filler into an acrylic resin is not necessarily satisfactory in balance between high light permeability and high light scattering property, in addition, it is insufficient in heat-resistance. A sheet material disclosed in JP-B No. 3-2188 has insufficient heat-resistance though it is excellent in balance between high light permeability and high light scattering property. A glutaric anhydride copolymer disclosed in JP-A No. 49-85184 has no light scattering property thought it is excellent in heat-resistance and light permeability.
However, these prior arts have respective problems.
The polymer having low birefringence has defects that since the number-average molecular weight Mn of the polymer is low, it has significantly lowered mechanical strength and usual injection molding is difficult though improved heat-resistance and hygroscopic property are obtained. Further, since the ester group in this copolymer is a cycloalkyl group having 3 to 8 carbon atoms, it is necessary to increase pyran polymer content for enhancing heat-resistance thereof. Namely, also in this case, polymer strength decreases. That is, conventional polymers having low birefringence as described above are not fully satisfactory as optical disks, lenses and light transmission materials.
Accordingly, an object of the present invention is to provide a methacrylic molding material which is excellent in transparency, heat-resistance, low hygroscopic property and mechanical strength and has low birefringence.
Regarding lenses for laser beam printers, since polycarbonates have high birefringence, and even though they have excellent heat-resistance and hygroscopic property, they have a problem that when they are used as materials for lenses of laser beam printers, the projected light is dispersed. In general, birefringence is required to become lower with progress of improvement in ability of a lens, and it is not practical to improve heat-resistance and hydrophilic properties at the expense of birefringence.
Therefore, another object of the present invention is to provide a lens for a laser beam printer having excellent in balance in heat-resistance, hydrophilic properties and birefringence.
Regarding optical disk substrate and optical disks, PMMA has problems of low heat-resistance and is highly hydrophilic, though is has low birefringence. On the other hand, polycarbonates has a problem of high birefringence though they are excellent in heat-resistance and water absorption, and birefringence thereof should be lowered for responding to recent increased density of optical information.
Therefore, another object of the present invention is to provide an optical disk substrate and optical disk having good balance of birefringence, heat-resistance and hydrophilic properties.
Regarding plastic optical fibers, the materials of the above-described technologies 1) to 5) have a problem that light transmission loss thereof is higher by far as compared with POF containing MMA as a core material. Also, the materials of the above-described technologies 1) and 2) have a problem of large change with passing of time under high temperature.
Further, when use in movable bodies such as automobiles is taken into consideration, it is expected that bending parts increase since cables have to be arranged in restricted space.
High light transmission loss of a fiber itself is disadvantageous since transmission loss increases by bending of the fiber in general, and improvement of heat-resistance at the expense of transmission loss as described in the above-described technologies 1) to 5) can not be admitted to be satisfactory.
Therefore, another object of the present invention is to provide POF having remarkably improved heat-resistance and revealing smaller increase in transmission loss as compared with PMMA.
Further, regarding light-inductive materials, there are problems that polycarbonates can not be recognized to have sufficient heat-resistance though they have excellent transparency and high heat-sensitive temperature, therefore, the light-inductive materials have to be made into forms which do not reach such a high temperature, or special means should be provided to prevent over heating, and the like. Polycarbonate also have a problem regarding weather-resistance.
Therefore, another object of the present invention is to provide a light-inductive material excellent in transparency, weather-resistance and heat-resistance.
Further, regarding modified polycarbonate sheets and production thereof, setting of resin temperature in an extruder is difficult since the glass transition temperature (Tg) of a methacrylic resin is lower by about 50.degree. C. as compared with that of a polycarbonate resin and the optimal melting temperatures of both resins have a difference of about 50.degree. C. in this method. Further, suitable roll temperatures of both resins have a difference of 30 to 50.degree. C., therefore, when the roll temperature is set for a polycarbonate resin, a methacrylic resin is decomposed or has deteriorated releasing property from a roll leading to generation of scar on the surface of a sheet due to releasing failure. On the other hand, when the roll temperature is set for a methacrylic resin, there occurs a problem of generation of warping in a polycarbonate.
Therefore, another object of the present invention is to provide a modified polycarbonate sheet having improved weather-resistance in high productivity and low cost.
Further, regarding a resin composition, an object of the present invention is to provide a resin composition which has no defects as described above in optical properties and weather-resistance and has excellent impact-resistance and heat-resistance. Another object of the present invention is to provide a thermoplastic resin composition which has excellent heat-resistance, has weather-resistance corresponding to that of polymethyl methacrylate, and has excellent oxidation-degradation-resistance.
Also, regarding lamp lenses, polycarbonates have a problem of relatively higher specific gravity among plastics. Further, polycarbonates can not be recognized to have sufficient heat-resistance, therefore, specific forms have to be made so that lenses do not reach such a high temperature, or special means should be provided to prevent over heating, in practical application. Namely, there has been known no lamp lens composed of a thermoplastic polymer which has excellent transparency and weather-resistance, low specific weight, and high heat-resistant temperature.
Further, regarding a sheet molded material having light scattering property, an object of the present invention is to provide a molded article which has extremely excellent total light permeability and light scattering property and has excellent heat-resistance.